Transformers are used in electronic systems for transforming impedances, interfacing balanced and unbalanced components, power splitting and combining, signal inversion, direct current blocking and delay lines. Four exemplary types of transformers are magnetic flux linkage transformers, lumped-component transformers, resonant transmission line transformers (TLT), and non-resonant TLTs. Non-resonant TLTs exhibit lower insertion loss and have wider bandwidth than magnetic flux linkage transformers and lumped-component transformers at radio frequencies (RF). The length of non-resonant TLTs is typically less than λmin/8 where λmin is the wavelength of the highest frequency in the operating bandwidth of the TLT. The length of the resonant TLTs is typically λcenter/4 where λcenter is the wavelength of the center frequency of the operating bandwidth of the TLT. Therefore, non-resonant TLTs are typically physically smaller than resonant TLTs. Smaller size is generally preferred. For at least these reasons, non-resonant TLTs exhibit preferred performance as compared to resonant TLTs.
However, non-resonant TLTs are difficult to integrate into printed circuit boards. Non-resonant TLTs have 1 or more 2-wire transmission lines. Each 2-wire transmission line is typically less than λmin/8. Criteria for successful operation of a non-resonant TLT include that: (1) at least one of the 2-wire transmission lines should exhibit very high common-mode impedance, and (2) the differential-mode impedance should be some function of the source and load impedances and of the TLT configuration.
Optimal differential-mode impedance is typically not difficult to realize. However, a very high-common mode impedance is difficult to realize with strip line circuitry. The achievable common mode impedance determines the lowest operating bandwidth of the non-resonant TLT. Very high common mode impedance has been achieved over a large bandwidth by wrapping a transmission line around a ferrite core or by use of ferrite beads. However, these types of construction are not compatible with the integration requirements of modern telecommunications radios
Attempts have been made to design non-resonant TLTs with the use of strip line to permit integration on printed circuit boards. FIG. 1 is a diagram of a Guanella TLT 100, which is a 2-wire transmission line that may exhibit a high common mode impedance. In FIG. 1, a first conductor 18 and a second conductor 20 are substantially physically parallel. FIG. 2 is an inductive circuit that represents a Guanella TLT of which the configuration of FIG. 1, is but one example. Note that the input and output ports are indicated by reference numerals 1 through 4. FIG. 3 shows a second order Guanella TLT 160 and FIG. 4 is an inductive circuit that represents a second order Guanella TLT of which the configuration of FIG. 3 is but one example. The second order Guanella TLT 160 has a first set of two substantially physically parallel conductors 18 and 20. TLT 160 also has a second pair of substantially physically parallel conductors 22 and 24. The conductor 18 provides an input 26 and the conductors 18 and 24 provide an output 28. Conductors 18 and 22 are coplanar and conductors 20 and 24 are also coplanar, but in a different plane.
FIG. 5 is a Ruthroff TLT 30. FIG. 6 is an inductive circuit that represents a Ruthroff TLT of which the configuration of FIG. 5 is but one example. The TLT 30 includes two edge coupled conductors 32 and 34. The Ruthroff configuration of FIG. 6 typically has a lower frequency limit of operation than the Guanella configuration of FIG.2. Note that both the Ruthroff and Guanella TLTs can be constructed with edge coupled conductors.
Combinations of the basic elements of FIGS. 2, 4 and 6, may be used as transformers, baluns, combiners and splitters. However, a high common mode impedance over a large bandwidth is difficult to achieve with the basic Guanella and Ruthroff elements when implemented with strip line techniques. When high common mode impedance is not realized over a large bandwidth, the resultant bandwidth of the non-resonant TLT will not be large.
Techniques for increasing the common-mode impedance of a 2-conductor stripline have been applied to non-resonant TLTs. A first technique is based on the relationship that maximizing the common-mode impedance of coupled transmission lines corresponds to maximizing the coupling coefficient between these lines. Increased coupling is achieved by vertically stacking two physically parallel conductors, which is known as broadside coupling and is shown in FIG. 1.
In addition to broadside coupling, various strip line parameters can be optimized to permit stronger coupling, such as: increasing the distance between the ground plane and the transmission lines, and decreasing the distance between the transmission lines. A problem with this technique is that the impedances of the common- and differential modes cannot be chosen independently. So maximizing the common-mode impedance does not ensure that constraints on the differential-mode can be met.
Another technique involves increasing the inductance of the common-mode currents by wrapping the two parallel conductors into a spiral shape (planar or multilayer), like an inductor. This wrapping is shown in FIG. 1 for broadside-coupled lines, and in FIG. 5 for edge-coupled lines. The impedance of an inductor is: j2πfL, (j=√−1, f is frequency, L is inductance), which shows that increasing the inductance increases the impedance. The differential-mode currents are not significantly affected by the wrapping of the lines. The drawback of this technique is that to achieve a high impedance for low frequencies, then the inductance must be very large. To increase the common-mode inductance of a pair of spiraled parallel conductors, the number of turns and/or outer circumference of the spiral must increase. Both of these modifications increase the length of the non-resonant TLT which increases the insertion loss at all frequencies.
Therefore, what are needed are non-resonant TLT configurations that increase common mode impedance and that are manufacturable using printed circuit techniques.